Physical and chemical processing methods for producing nanocrystal films with properties by design

Abstract

As quantum dot (QD) synthesis techniques and device architectures advance, it has become increasingly apparent that new ways of connecting QDs with each other and the external environment are required in order to realize the considerable potential of QDs for optoelectronic applications. Throughout my PhD studies at Cornell, I have worked to establish the scientific and engineering foundation for processing techniques to produce designer materials from QD building blocks. Specifically, I have investigated two general processing methods, thermal annealing and solution based chemical treatments to remove or replace the insulating native ligands and produce electronically coupled thin films. In a series of studies on thermal annealing of QD films across 10 orders of magnitude in time, I show how nonequilibrium laser annealing over ns and μs can be used to precisely control the structure of QD thin films to increase electronic coupling while maintaining quantum confinement, how in situ studies of QDs arranged in a periodic nanoreactor can shed light on QD fusion at the second to minute time scale, and how spatial temperature gradients during nonequilibrium laser annealing can be exploited to reveal that QD sintering is a thermally activated process with a constant activation energy over two orders of magnitude of QD growth rate. My work on chemical processing of QD films focuses on low temperature solution processing methods. I demonstrate how simultaneous cation and ligand exchange at the surface of QDs can electronically couple and passivate QD films in a single step, leading to a 4 fold increase in Förster resonant energy transfer rate and order of magnitude reduction in trap density. Through controlled removal of ligands and post assembly QD growth, I show how building epitaxial bonds among QDs in a long range ordered assembly can lead to a ~3 order of magnitude increase in the mobility of carriers in QD films. This work is an illustration of how detailed understanding of the processing-structure-property relationships in QD assemblies over multiple length scales can produce functional thin films with properties by design. Further advances that build on this work and others will take full advantage of the unprecedented flexibility provided by the size tunable properties of QDs to expand the periodic table into another dimension and drive materials innovation.